Urethane coatings are defined as coatings that contain urethane (--NHCOO--) or urea (--NHCONH--) groups. They are reaction products of isocyanates and hydroxyl functional resins. Depending on the hydroxyl components, urethane coatings can be divided into acrylic-urethanes, polyester-urethanes, polyether-urethanes, and many others. Properties and applications of urethane coatings depend largely on the hydroxyl components. For example, polyether-urethane coatings have excellent flexibility but poor weathering stability and chemical resistance, while acrylic-urethane coatings have excellent weathering stability and chemical resistance but low flexibility.
Urethane coating formulations can also be categorized into two-component (two-pack) and one-component (one-pack) systems. Two-component urethanes consist of a hydroxyl component and an isocyanate (NCO) component. These two components are packed separately and mixed in a desired ratio prior to application. One-component urethanes include urethane lacquers (solutions of thermoplastic urethanes), uralkyds, blocked urethanes, and moisture-cure urethanes. An advantage of one-component urethanes is their convenience of application. Moisture-cure urethane formulations contain in a single package an NCO-terminated resin, a solvent, and, optionally, a catalyst. When the coating is applied on a surface, the solvent evaporates, and the NCO groups react with moisture in the air and form urea linkages to crosslink the resin. Since they are cured by moisture in the air, their utility is limited to thin films. Another disadvantage of prior art moisture-cure urethane coatings is that the NCO-terminated resins have higher molecular weights, therefore higher viscosities than those of two-component urethane coating formulations. This limits the growth of moisture-cure urethane coatings because of the higher solvent demand and the resulting higher VOC (volatile organic compound) contents. Moisture-cure urethane coatings are usually polyether-urethanes. Because of the poor weathering stability and chemical resistance of polyether polyol systems, these moisture-cure urethane coatings are used primarily as primers and base-coats for surfaces of metal, wood, and concrete structures such as water tanks, pipes, bridges, and decks.
Phenolic resins, though widely used in other coatings, have found limited use in urethanes because phenolic hydroxyl groups are not sufficiently reactive with isocyanates. Modifying urethane coatings with phenolic resins has been attempted because they can potentially improve chemical resistance of urethanes, particularly moisture-cure urethanes. For example, U.S. Pat. No. 4,539,345 discloses moisture-cure urethanes based on blends of an NCO-terminated polymer, a terpene-phenolic resin, and a silane compound.
Alkoxylating phenolic resins changes phenolic hydroxyls into aliphatic hydroxyls and makes it possible to use phenolic resins in urethanes. For example, U.S. Pat. No. 4,167,538 teaches the preparation of a resinous (i.e., solid) alkoxylated phenolic resin. The resin has a softening point (for polymers, the softening point is usually about 50.degree. C. below the melting point) within the range of 30.degree. to 150.degree. C., preferably from 40.degree. to 80.degree. C. However, the alkoxylated phenolic resin is not suitable for use in moisture-cure urethane coatings because it is too rigid and has a high solution viscosity.
Other alkoxylation techniques are known. For example, U.S. Pat. No. 5,679,871 teaches alkoxylating phenolic resins with alkylene carbonates rather than alkylene oxides. The advantage of using cyclic carbonates such as ethylene glycol carbonate or propylene glycol carbonate is that the alkoxylated phenolic resins do not contain long oxyalkylene chains; unlike alkylene oxides, alkylene carbonates do not polymerize during the alkoxylation. Alkoxylated phenolic resins with short oxyalkylene chains have high glass-transition temperatures or softening points.
Formulating urethane coatings from alkoxylated phenolic resins is also known. For example, WO 97/19972 teaches how to prepare a two-component urethane coating from a propoxylated phenolic resin. However, the coating has a low solids and high VOC contents.
It is unknown to formulate moisture-cure urethanes from alkoxylated phenolic resins, especially from liquid alkoxylated phenolic resins that have long oxyalkylene chains. We have surprisingly found that moisture-cure urethane coatings prepared from liquid alkoxylated phenolic resins have extremely low solvent demand and, therefore, very low VOC contents. In addition, the coatings have excellent chemical resistance compared to the conventional moisture-cure urethane coatings.